The present invention relates to a piston unit adapted to be controlled by liquid pressure and, more particularly, to a small-sized piston unit having a piston reciprocatively displaceable in a cylinder filled with liquid, wherein the piston is subjected to different fluid resistances caused by the liquid when it makes forward and backward movements over a predetermined stroke within the cylinder, so that the piston may displace at different stroke speeds in its forward and backward movements.
Generally, in the design and construction of this kind of device, it is essential to avoid leakage of the liquid filling the cylinder, e.g. water, oil or the like, to the outside of the device. Particularly, it is important to prevent the liquid from leaking through the sliding portion between the cylinder and the piston rod which is connected to the piston.
A typical conventional measure for preventing the leakage of the liquid is to provide sealing members such as "O" rings in the portion of the cylinder where it slideably embraces the piston rod. With this measure, however, it is impossible to perfectly seal the internal liquid, because the pressure of the liquid in the cylinder is increased considerably due to the reciprocation of the piston in the cylinder thereby causing the liquid to leak through the portion of the cylinder slideably embracing the piston rod.